The present invention relates to frequency transposition and, more particularly, to radio frequency transposition such as in a mobile telephone. In mobile telephones, radio-frequency circuits make wide use of frequency transposition devices (mixers), both on transmission and reception of information signals.
During transmission of an information signal, the purpose of a frequency mixer, which in this instance is a frequency raising circuit, is to transpose the information signal into a baseband signal around a transmission carrier signal. On reception of the information signal, the frequency mixer is a frequency lowering arrangement.
The frequency transposition function is critical, both on account of its conventional limitations (e.g., linearity, power consumption, noise factor), and also because of the isolation of the signal originating from the local oscillator towards the output signal from the mixer. The residual quantity of a local oscillator signal in the output signal from the mixer must be minimized to make it easier to recover the carrier signal.
As an illustration, if the input signal is at a frequency between 100 and 400 MHz, and to produce an output signal around 1 or 2 GHz, the local oscillator signal, which is locked onto a frequency of about 1 or 2 GHz, is close to the synthesized carrier signal at the output. The subsequent filtering of the local oscillator signal is often difficult, at least at an acceptable cost.
The leakage of the local oscillator signal to the output signal, which is to be minimized, is called a structural leakage, i.e., it is intrinsic to the mixer. Another cause of leakage that is conventionally encountered occurs between the input of the local oscillator on the chip containing the mixer, and the output of the mixer by way of the parasitic elements of the package. This leakage can be easily addressed with modern packaging.
FIG. 1 illustrates the structure customarily used for frequency transposition devices. In the top part of FIG. 1, the reference DTF designates a frequency transposition device, or mixer, which is also referred to herein as a frequency raiser. The frequency raiser includes an input terminal BE for receiving an input signal at an intermediate frequency IF, which may be 200 MHz, for example, and an input terminal BO for receiving the local oscillator signal LO, which may be 2 GHz, for example. An output terminal BS delivers the output signal whose frequency spectrum exhibits a line at the frequency LOxe2x88x92IF and a line at the frequency LO+IF. The dashed arrow illustrates the structural leakage of the oscillator signal LO to the output signal.
The structure customarily used for these mixers is a differential GILBERT type, as illustrated in the bottom part of FIG. 1. More precisely, such a structure comprises a differential transducer block BTC for converting the input signal (voltage) present on the terminals BE into a differential current. This block BTC also comprises a stage forming the output stage, and includes a differential pair of transistors T1 and T2, whose respective bases are linked to the input terminals BE by two capacitors C6 and C7. The collectors of the two transistors T1 and T2 of the output stage form the output terminals of this transducer block BTC. Alternatively, the block BTC may comprise several stages.
A resistor RP, contributing to defining the transductance value of the block, is connected between the emitters of the transistors T1 and T2. The transistors T1 and T2 are biased by biasing means MPL which includes two current sources SC1 and SC2 connected respectively between ground and the two terminals of the resistor RP. The biasing means MPL also comprises two base resistors RB1 and RB2, both connected to a voltage source ST. This connection forms a return path for the base currents of the transistors T1 and T2.
At the output of the transconductor block BTC, i.e., the collectors of the transistors T1 and T2 of the output stage, a current switching block BCC is connected for shunting the current alternately to one of the two output terminals BS at the frequency of the local oscillator signal LO received at the terminals BO. This block BCC conventionally comprises two pairs of transistors Q3, Q4 and Q5, Q6.
Each resistor ZL, connected between the output terminals BS of the block BCC and the supply Vcc, represents the output load of the mixer DTF. The transconductor BTC, which is formed by transistors T1 and T2 and resistor RP, and which is also used to define the transconductance of the block BTC, converts the power or the voltage applied to the input BE into a differential current. This differential current is an image, assumed linear, of the input signal. This linear signal is then chopped by a nonlinear square function (+1, xe2x88x921, +1, xe2x88x921 . . . ) carried out by the double switch BCC at the frequency of the signal LO. The double switch acts as a dynamic shunter of the current. The output signal is gathered at the terminals of the differential load 2ZL.
In the first instance, it is shown that the output is a balanced structure that generates an output signal free of the local oscillator signal residual. However, this absence of the local oscillator signal in the output signal relies on a perfectly differential structure. In practice, a residual quantity of the local oscillator signal exists in the output signal by reason of an imperfectly differential structure. Stated otherwise, the symmetric elements of the structure do not posses identical characteristics after fabrication on silicon, i.e, they are not matched.
A common cause is poor matching of the parasitic capacitances of the current switch and/or of the two output load impedances ZL. The present invention provides an approach to this problem by adopting a radically different approach from that of the prior art, which reduced the lack of dynamic matching of the structure, i.e., the lack of capacitive and resistive matching.
The dominant cause of the lack of matching of the differential structure in not from a lack of dynamic matching, but from a lack of static matching, and more particularly, from poor matching of the quiescent currents of the differential structure.
An object of the present invention is to better match the quiescent or bias currents of the differential pair of transistors of the output stage of the transconductor block BTC. In other words, the object of the present invention is to better match the emitter currents of these transistors.
The biasing means conventionally used, such as illustrated in FIG. 1 under the reference MPL, exhibit numerous causes leading to an absence of matching. One of these causes results from the poor matching of the two current sources SC1 and SC2. Moreover, when the transistors T1 and T2 are biased to high current levels (a few mA to 10 mA) they easily develop offset voltages from 5 to 10 mV. The matching of the bias currents depend not only on the matching of the respective factors xcex2 of the transistors, but also on the matching of the respective offset voltages of these transistors.
Moreover, the base bias resistors RB1 and RB2 induce a lack of matching, which is very sensitive on account of the base current offset (poor matching of the DC gains). This is found in bipolar transistors, particularly in high speed devices. The orders of magnitude are such that the lack of matching of the output currents is from 5 to 10% if one assumes a lack of matching of the gains of the transistors of 10%. The situation may be worse when the matching of the gains is yet further degraded to around 20% or 30%, as is sometimes the case in high speed devices.
The invention therefore matches the quiescent currents of the structure to reduce the residual quantity of the local oscillator signal in the output signal from such a mixer. This is done by performing differential slaving of the quiescent currents of the output stage of the transconductor block accompanied by common mode control.
According to the invention, the transistors of the output stage of the differential transconductor block of the structure are biased by carrying out differential slaving of the emitter currents (for bipolar technology or source currents for MOS technology) of these transistors to a predetermined common mode current. That is, the difference of the values of the two emitter currents is slaved to zero while slaving the value of each of these currents to a predetermined value. The predetermined value is the value of the common mode current.
Stated otherwise, the differential slaving of the two emitter currents or source currents of the output stage of the transconductor block renders these two quiescent currents equal. Thereby, the matching of the two output currents, which are the two collector currents, then depends on the lack of matching xcex94xcex2 of the two transistors only in a ratio 1/xcex22. Thus, for usual values of xcex2 on the order of 50, and a ratio xcex94xcex2/xcex2 which even at the worst is on the order of 0.5, a matching of the collector currents of the transistors T1 and T2 to within 1% is obtained.
This being so, it is necessary, not only for the two bias currents to be equal, but it is also necessary for them to be equal to a common mode current set in advance. This is the reason why common mode control or slaving is also performed. The bias or quiescent currents are the emitter currents in the absence of a signal (intermediate frequency or radio-frequency) at the input of the transconductor block. The emitter currents in the presence of an input signal are different from the quiescent emitter currents.
The average of the signal received at the input is zero, and the variations in the emitter currents which are corrected by the slaving loop according to the invention are not the variations due to the input signal. These variation are from the process (lack of matching), and from temperature variations, etc. These variations which occur as soon as the mixer is energized, are corrected by the slaving loop with a time constant that is relatively large in relation to the temporary variations of the input signal.
Accordingly, within the meaning of the present invention and in the subsequent text, the input signal is ignored and the emitter or source currents, which are slaved are regarded even in the presence of input signal, are the bias or quiescent currents of the respective transistors.
According to one mode of implementation of the invention, the differential slaving is carried out by looping back directly or indirectly between the emitters (sources) and the bases (gates) of the transistors of the output stage of the transconductor block. A differential amplifier, preferably with transconductance (current output) whose input stage comprises two transistors having linked bases (gates), and the common mode current is fixed by applying a reference voltage to the linked bases (gates) of the transistors of the differential amplifier. This makes it possible to apply a relatively small voltage to the emitters of the transistors of the output stage of the transconductor block.
When the transconductor block comprises just one stage, which is both the input stage and the output stage, the looping back of the differential amplifier is direct. That is, the output of the amplifier is connected directly to the bases of the transistors of the transconductor block.
When the transconductor block comprises at least two mutually linked stages, i.e., at least one input stage and one output stage, the looping back of the amplifier is indirect. The output of the amplifier is connected to one of the electrodes of the transistors of the input stage of the transconductor block. The output is also consequently connected indirectly to the bases of the transistors of the output stage by way of the transistors of the input stage, and possibly by way of the transistors of the intermediate stages.
Such a mode of implementation thus also makes it possible to use just a single differential amplifier to carry out both the differential slaving of the emitter currents to the common mode current, and to fix this common mode current. Such an approach is distinguished from another approach, also possible, which would include using a differential amplifier to carry out the slaving of the emitter currents to the common mode current, and a separate slaving loop carrying out the slaving of the common mode. That is, the value of the common mode current is fixed.
It would be possible to apply the reference voltage to the linked bases (gates) of the transistors of the differential amplifier by using a reference voltage source directly. However, in this case, the value of the quiescent current depends on the base-emitter (gate-source) voltage drop of the transistors of the input stage of the differential amplifier. This voltage drop is not temperature-stable. Hence, it is preferable, in certain applications, to apply the reference voltage to the linked bases of the transistors of the differential amplifier from a reference current source and from a reference resistor mutually connected by a current mirror. The base of one of the transistors is connected to the linked bases of the transistors of the differential amplifier. The formula giving the bias current is then independent of a base emitter voltage drop of a bipolar transistor, or a gate-source voltage drop of an insulated-gate field-effect transistor.
Although it would be possible to bias the transistors of the input stage of the differential amplifier with a bias current emanating from a current source separate from the reference current source, it is preferable, for matching reasons, to bias these transistors with a bias current emanating from the reference current source. This bias current can then be equal to the reference current emanating from the reference current source, or else equal to a fraction of this reference current.
The subject of the invention is also a frequency transposition device of the type comprising a differential transconductor block for converting an input signal into a differential current, and a differential output stage with two transistors. A bias circuit biases the transistors of the output stage. A current switching circuit is controlled by a local oscillator signal, and is connected between the output stage of the transconductor block and the output terminal of the device.
According to a general characteristic of the invention, the biasing circuit comprises bias means for generating a predetermined common mode current, and slaving means for carring out a differential slaving of the emitter currents or source currents of the transistors of the output stage of the transconductor block to the common mode current.
According to one embodiment of the invention, the slaving means comprises a differential amplifier with transconductance comprising an input stage with two transistors whose bases are linked together, whose emitters are linked respectively to the emitters of the transistors of the output stage, and whose collectors are looped back directly or indirectly to the bases of the transistors of the output stage.
Moreover, the means for generating the common mode current comprise a voltage source delivering a predetermined voltage to the linked bases of the transistors of the input stage of the differential amplifier, as well as two bias resistors connected between ground and the emitters of the transistors of the output stage of the transconductor block.
According to a particularly advantageous embodiment of the invention, the reference voltage source comprises a reference current source delivering a predetermined reference current, a reference resistor connected to ground, and a current mirror connected between the reference current source, the reference resistor and the linked bases of the transistors of the input stage of the differential amplifier.
The subject of the invention is also applicable to a cellular mobile telephone, which comprises a frequency transposition device as defined herein.